1. Field of the Invention
The present invention relates to a process for the preparation of tert-C4–C8-alkyl esters of (meth)acrylic acid by reacting (meth)acrylic acid with an olefin of the formula where    R1 and R2, which may be identical or different, are methyl or ethyl and R3 is H, methyl or ethyl, in the presence of an acidic catalyst in homogeneous phase and isolating the ester from the reaction mixture.
2. Description of the Background
Tert-alkyl esters of (meth)acrylic acid are important starting materials for the preparation of polymers which are used, inter alia, as a component of coating dispersions, adhesives or coating resins. The preparation of tert-alkyl esters is carried out in general by an acid-catalyzed addition reaction of the corresponding carboxylic acids with isoolefins (Houben-Weyl, Methoden der Organischen Chemie, Vol. 8, 1952, page 534; U.S. Pat. Nos. 3,031,495 and 3,082,246). The catalysts used in the reaction mixture are soluble acids, for example mineral acids or alkanesulfonic or arylsulfonic acids (DE-A-12 49 857, U.S. Pat. No. 3,087,962, U.S. Pat. No. 3,088,969), or insoluble catalysts, such as acidic ion exchanger resins (U.S. Pat. No. 3,037,052, U.S. Pat. No. 3,031,495, DE-A-31 05 399, EP-A-268 999).
The reaction of the carboxylic acids with the isoolefins is carried out as a rule in conventional reaction containers or in columns (DE-A-11 28 428), the thorough mixing of the reaction mixture being effected by stirring or by the isoolefin stream passed in. Heat is removed in a conventional manner.
As a rule, the following difficulties arise during the preparation of tert-alkyl esters:                oligomerization of the isoolefins        polymerization of the (meth)acrylic acid or of the esters under thermal stress        cleavage of the tert-alkyl esters under the action of heat and/or in the presence of traces of strong acids        insufficient removal of the heat of reaction occurring in the strongly exothermic esterification reaction which may result in cleavage of the ester, oligomerization of the isoolefin and polymerization of the (meth)acrylic compounds.        
In the prior art, numerous attempts have been made to reduce or to prevent the difficulties encountered. For example, the tendency of the isoolefins to oligomerize can be reduced by decreasing the reaction temperature (U.S. Pat. No. 3,172,905), by the presence of water (U.S. Pat. No. 3,088,969) and by partial neutralization of the catalyst (DE-A 31 05 399). However, these measures have the disadvantage that the reaction rate is reduced.
The formation of the isoolefin oligomers can also be reduced by using gaseous isoolefins (DE-A-11 35 897). However, the disadvantage here is the required vaporization of the liquid isoolefin and the handling of large amounts of gas.
Owing to the abovementioned difficulties, the isolation of the tert-alkyl ester from the reaction mixture is also particularly important. Frequently, the acidic catalyst is extracted and/or neutralized in order to prevent cleavage of the tert-ester (U.S. Pat. No. 3,172,905; U.S. Pat. No. 3,037,052; DE-A-11 28 428 and DE-A 11 35 897). However, this method causes environmental pollution and is uneconomical. Moreover, when amines are used for neutralizing the catalyst there is the danger that the polymerization of the (meth)acrylic acid and/or of the tert-ester will be promoted (cf JP 60 130 546). DE-A 12 49 857 proposes a mild distillation at from 30 to 80° C. and from 0.1 to 100 mbar for separating off the catalyst (sulfuric acid). The bottom product obtained is recycled to the reactor and the top product, a mixture of ester and diisoolefin, is further worked up. However, the fact that in this method the top product is nevertheless not completely free of catalyst is disadvantageous.
The strong tendency of (meth)acrylic compounds to polymerize, especially at relatively high temperatures, is known. Particularly in the purification by distillation these compounds are generally exposed to temperatures which can readily initiate an undesired polymerization. This results in soiling of the apparatuses, blockage of pipes and pumps and coating of column trays and heat exchanger surfaces. Cleaning of the plants is a complicated, expensive and environmentally polluting operation, and the availability of the plants is thus greatly reduced. It is important in this context that acetic acid is formed as a by-product in an amount of up to 1% by weight in the preparation of (meth)acrylic acid, for example by gas-phase oxidation of propene or acrolein or of isobutene or methacrolein over metal oxide catalysts. However, acetic acid is a troublesome component which has to be separated off before the (meth)acrylic acid is used. Owing to the small boiling point difference and the strong tendency of the (meth)acrylic acid to polymerize, however, removal of the acetic acid is difficult and expensive (U.S. Pat. No. 3,844,903 and DE-A 21 64 767), so that the use of substantially acetic acid-free (meth)acrylic acid is economically disadvantageous.
In order to avoid or reduce the polymer formation in (meth)acrylic compounds, as a rule polymerization inhibitors, for example phenothiazine, hydroquinone, hydroquinone monomethyl ether, p-nitrosophenol or tert-butylpyrocatechol or mixtures thereof, are used, if necessary with the simultaneous action of air (DE-A-11 35 897, DE-A-12 49 857, EP-A-0 24 534 and DE-A-29 31 553). Complete prevention of polymer formation is however not possible by the known methods.